1. Field of the Invention
The present invention relates to a vacuum pump system for providing a vacuum state, and more specifically, to a vacuum pump system which can collect and absorb airborne dust.
2. Description of the Related Art
Various vacuum pump systems have been utilized to keep a closed container a in low or medium vacuum state. When the gas extracted from the closed container contains dust particles, a dust filter which can collect the dust particles is provided in the way between the closed container and the vacuum pump to maintain the performance of the vacuum pump, therefore, only pure gas will be absorbed in the vacuum pump.
FIG. 6 illustrates the configuration of a vacuum pump system for a single-crystal semiconductor pulling apparatus. The substrate material of a semiconductor device consists of single-crystal silicon of high purity. The single-crystal silicon can be fabricated by methods such as the CZ method. The CZ method includes filling a quartz crucible with polycrystal material, heating the polycrystal material to the melting point by heaters located around the quartz crucible, immersing in the melt a seed which is fixed on a seed holder, and pulling up the seed holder to form the single-crystal material. When the polycrystal material is melted, the melt will react with the crucible made of quartz to generate SiO.sub.x. The amorphous SiO.sub.x (amorphous silicon) will gather or adhere on the wall surface, the surface of the pulled single-crystal material, or the wall surface of the furnace. This substance, if melted into the as-grown single-crystal material, will change the phase of the single-crystal material, thus affecting the fabrication efficiency. In order to solve this problem, high purity inner gas is introduced above the semiconductor pulling apparatus to bring the amorphous silicon away, and then is expelled out through the bottom or lower portion of the furnace.
In the aforementioned semiconductor pulling apparatus, the inner gas can be absorbed by a vacuum pump system using mechanical press and oil pressure pump, thus keeping the pressure inside the furnace in a low or medium vacuum state. However, if the amorphous silicon adheres to the rotator of mechanical press, the rotator will be blocked as a result of the narrow spacing (about 0.1 to 0.5 mm) between the rotator and its cover. Moreover, the lubricant in the oil pressure pump will have a higher viscosity if mixed with the amorphous silicon. Therefore, the pump cannot operate normally and may even stop pumping.
The arrangement illustrated in FIG. 6 is provided for preventing the aforementioned problem in the vacuum pump system used in the conventional semiconductor pulling apparatus. That is, a dust filter 21 consisting of a number of metal meshes is provided in the midway of a pipe 20 which connects the semiconductor pulling apparatus 1 and the vacuum pump. The harmful amorphous silicon can be collected by the filter 21, and then the inner gas is absorbed by the mechanical press 9 and the oil pressure pump 22. As shown in the figure, element 4 is a manual valve, element 5 is an emergency auto valve, element 6 is a bellows expansion joint, element 7 is an auto valve, and element 23 is a oil gas collector.
Nonetheless, in order to remove the dust particles completely, the mesh sizes of the dust filter 21 must be as small as possible, thus increasing the pressure difference between opposite sides of the filter. This problem can be solved by improving the pumping ability of the vacuum pump or increasing the effective area of the dust filter. However, the two solutions will increase the apparatus dimensions and manufacturing cost.
Furthermore, the amorphous silicon will keep accumulating on the dust filter as the semiconductor is continuously pulled. Therefore, the pressure difference between opposite sides of the filter increases during the semiconductor pulling process. Since the furnace pressure in the beginning of a pulling process is much lower than that at the end of the process, the quality of the single-crystal varies from its top portion to the end portion. In order to prevent an increase in the furnace pressure, the dust filter must be changed or cleaned whenever a pulling process has been done. This additional maintenance task wastes a lot of time.